Getter ion pump method and apparatus



Sept. 22,1964 v jR.`| .YJEPsEN 3,149,774

GETTSR'ION PUMP METHOD AND APPARATUS Filed Jan.y 27, A1961 y l l l I o5MM United States Patent O 3,149,774 GETTER N PUMP METHOE) AND APPARATUSRobert L. Jepson, Los Altos, Calif., assigner to Varian Associates, PaloAlto, Calif., a corporation of California Filed Jan. 27, 1961, Ser. No.85,355 16 Claims. (Cl. 230-69) The present invention relates in generalto glow discharge devices and more particularly to glow dischargedevices having a high throughput capacity.

Heretofore electrical cold cathode gas discharge vacuum pumps have beenbuilt having for `their principle of operation the establishment of aglow discharge within the interior of au open ended tubular anodedisposed between and spaced from two cathode .plates and having amagnetic eld threaded through the anode. Positive ions produced by theglo-w discharge are directed against the cathode plates and theimpinging ions produce sputtering of a reactive cathode material. Thesputtered material is collected upon the interior surfaces of the pumpwhere it serves to entrap molecules in the gaseous state coming inContact therewith. In this manner the gas pressure within a vesselenclosing the cathode and anode elements is reduced.

The operating performance of these pumps is determined largely by theirpumping speed which is the volume rate at which gases Aare being removedfrom the vacuum system. The pumping speed is influenced by several`factors such as' the particular design and size of the pumpingelements, the magnitude of ion current produced which is, in turn,dependent upon the applied voltage, etc. K

However, in certain types of vacuum applications the actual pumpingspeed of a vacuum pump is of less importance than the pumps throughputcapacity which is determined by multiplying the pressure at which thepump is operating times its pumping speed `at that particular pressure.Throughput is therefore a measure of the pumpsactual molecular rate ofgas removal. A prime example of an application in which throughputcapacity is of greater importance than pumping speed is vacuum tubeprocessing in which large quantities of cathode conversion gases areproduced at certain times during the process thereby requiring a highmolecular gas removal rate at these times while requiring a relativelylow pumping speed to attain v the desired ultimate pressure in the tube.

ly Heretofore the maximum throughput obtainable with pumps of a givensize has been limited by a number of considerations. Present pumps havea substantially con,- s'tant pumping speed in the low pressure regionbetween about 108104 mm. Hg so that a pumps throughput will be directlyproportional to operating pressure in this region. A maximum throughputwill be reached, however, in the pressure range between l0*A1 and 103mm. Hg for a number of reasons. It is in this pressure region that animpedance transition of the glow discharge takes place in present coldcathode pumps' whereby the impedance changes from `a high to acomparatively low value causing an increase in ion current and areduction in voltage across the glow discharge. This reduction involtage allows the glow discharge to become unconiined which iddfiPatented Sept. 22, i954 FPice greatly reduces the pumps pumping speedand thereby its maximum throughput.

Early attempts to enhance a pumps throughput capacity by increasedvoltages across the glow discharge failed to achieve satisfactoryresults. lt was not clear whether these failures resulted fromfundamental properties of the cold cathode gas discharge or were ratherdue to effects arising from power dissipation heating of the pumpelectrodes. Nor was it known whether power dissipation heating of thepump electrodes produced any change in the fundamental properties of thecold cathode gas discharge, especially in the exceedingly compleximpedance transition region. High power dissipation requirements inother types of electrical equipment can sometimes be met by cooling ofvarious kinds and some cooling has been tried in the electrical vacuumpump tield. For example, the `application of cooling means to theexterior of electrical vacuum pump envelopes has been tried. Theseapplications, however, are very inefficient because of the highinsulation properties of the vacuum which exists between the coolingmeans and the pump electrodes.

Cooling has also been utilized in cathode emitter pumps although forentirely different reasons. The intense heat generated by thermalemitters results in extensive outgassing from the walls of these pumpspreventing the attainment of high vacua. Cooling is primarily used,therefore, to prevent outgassing rather than to dissipate power producedby a glow discharge.

These early cooling attempts, being very inefficient, did not establishwhether a dependent relationship existed between eicient cold cathodepump electrode cooling and the characteristics of the cold cathode gasdischarge. Therefore, it was not apparent that the provision of anetiicient power dissipation means in a cold cathode vacuum pump to allowhigher working voltages would necessarily produce greater maximumthroughputs. It seemed quite possible that the use of adequate powerdissipation means, although permitting higher pumping speeds, might alsocause the glow discharge impedance transition to take place at lowerpressures than was previously the case. The overall effect of such anoccurrence could have resultedin a negligible increase or possibly evena decrease in aY maximum throughput obtainable in a given size pump.

It has been found, however, that the use of novel cooling means in acold cathode pump has resulted in a glow discharge impedance transitionat even higher pressures than those at which the transition took placewithout such cooling. This phenomenon, together with the greater pumpingspeeds achieved by higher working voltages, has resulted in maximumthroughput capacities some ten times larger than could be previouslyobtained with pumps of a given size.

It is, therefore, the object of the present invention to provide anovel, improved, compact cold cathode vacuum pump having high throughputcapabilities, and to provide a novel method for cooling a cold cathodevacuum pump so as to greatly enhance its throughput capacities.

One feature of the present invention is the provision of cooling meansfor a cold cathode vacuum pump in intimate contact with its reactivecathode thereby allowing higher power dissipation and greater throughputcapacities.

Another feature of the present invention is the provision of coolingmeans for a cold cathode vacuum pump cornprising an elongatedheat-conducting member in intimate contact with its reactive anode overa substantial part of its length thereby allowing higher powerdissipation and greater throughput capacities.

Another feature of the present invention is the provision of coolingmeans of the above types which allow a minimum gap between the polefaces of the pump.

Still another feature of the present invention is the provision for anovel method for cooling a cold cathode vacuum pump.

Other features and advantages of the present invention will becomeapparent upon a perusal of the specification taken in connection withthe accompanying drawings wherein,

FIG. 1 is a plan view partly in cross section of a novel electricalvacuum pump of the present invention,

FiG. 2 is an end view partly in cross section of the structure of FIG.1,

FIG. 3 is a graph showing throughput vs. pressure for the pump structureof FlG. 1,

FIG. 4 is a graph showing pumping speed vs. pressure for the pumpstructure shown in FIG. l, and

FiG. 5 is a plan view partly in cross section of another embodiment of anovel electrical vacuum pump of the present invention.

Referring now to FIGS. l and 2 a rectangular cupshaped member 1@ isclosed off at its flanged open end by a rectangular closure plate 6welded about its periphery to the flanged portion of cup-shaped member16 thereby forming a rectangular vacuum compartment '7. The rectangularclosure plate 6 is provided with a pair of apertures which are closed bya recessed cylindrical ,chamber S and an outwardly extending cylinder 9circumferentially supporting a cup-shaped member 11 having its open endfacing away from the closure plate e.

A rectangular cellular anode 12, of, for example, titanium, is carriedwithin the vacuum compartment 7 upon the end of a conductive rod 13which extends outwardly of the rectangular vacuum compartment 7 throughan aperture in the recessed cylindrical chamber S. The conductive rod 13is insulated from and carried by the vacuum compartment 7 through theintermediaries of annular insulator frames 141, 15 and 16 andcylindrical insulator 17. The free end of the rod 13 provides a terminalfor applying a positive anode voltage with respect to two substantiallyrectangular cathode plates 13 made of reactive material.

The cathode plates 1S are spaced apart at their corners by the bands 19and are supported from the closure plate u by the U-shaped support 21and `associated pin 22 so as to be mechanically locked in positionsubstantially parallel to and spaced from the anode 12. In theembodiment shown, the side wall of the vacuum compartment "i oppositethe closure plate 6 is apertured to receive the hollow conduit 23 whichmay be of any convenient inside diameter commensurate with the desiredpumping speed. The hollow conduit tube communicates with any desiredvacuum system and is provided with a suitable mounting ilange 24.

A horseshoe-shaped permanent magnet having a pair of pole pieces 26 ispositioned with respect to the rectangular vacuum compartment 7 suchthat the magnetic held of the magnet 26 threads through the individualcellular elements of the anode 12 in substantial parallelism t0 thelongitudinal axes thereof.

The continuous copper tube 2S is shaped to form a pair of rectangleswhich are brazed to the edges of the cathode plates 1S on their surfacesfacing the anode 12 so as to be enveloped by the vacuum compartment 7.The positioning of the copper ture 25 on the anode side of the cathodeplates 18 rather than between them and .the vacuum compartment 7 oroutside the vacuum compartment makes it unnecessary to either enlargethe gap between the magnet pole pieces 26 or increase the overall sizeof the pump. This is especially important from the point of View ofminimizing the size, weight, and cost of the magnet necessary to producea magnetic iield of desired strength. The ends of the copper tube 25 eX-tend out of the vacuum compartment 7 through a pair of apertures in thecup-shaped member 11. The dimensions of the rectangles formed by thecopper tube 25 are larger than those of the `anode 12 so as to be`outside the glow discharge paths between the anode and the cathodeplates. Thus, the copper tube 25 provides a path for a cooling iluid inintimate contact with the reactive cathode plates '18. This intimatecontact is extremely important since the high insulation properties of avacuum will allow even the smallest separation'between coolant and thereactive cathode material to greatly reduce cooling efliciency.

In typical operation of this device, a positive potential of .4-10 kv.or more is applied to the anode 12 Via conductive rod 13. The vacuumcompartment 7 and the cathode plates 18 are preferably operated atground potential 4to reduce hazard to operating personnel. The appliedpotentials produce a region of intense electric iield between thecellular anode 12 and the cathode plates 18. This electric eld, actingin combination with the magnetic eld, produces a breakdown of gas withinthe pump resulting in a glow discharge within the cellular anode 12 andbetween the anode 12 and the cathode plates 18. The glow dischargeresults in positive ions being driven into the cathode plate 1S toproduce ydislodgment of reactive cathode material which is therebysputtered onto the nearby anode 12 to produce gettering of molecules inthe gaseous state coming in contact therewith. In ythis manner, thepressure within the vacuum compartment 7 and, therefore, structurescommunicating therewith is reduced.

FIGS. 3 and 4 are graphs showing certain operating characteristics ofthe embodiment of the present invention shown in FG. l. Curve A is aplotof pumping speed in liters per second vs.,pressure in mm. Hg for acoldcathode vacuum pump using cathode Acooling means as described above.Curve B is ya corresponding plot of throughput in micron liters persecond vs. pressure in mm. kHg under the same operating conditions.Curves C and D are estimated curves of the same operatingcharacteristics for the same pump without cathode cooling means and runwith a power supply of appropriately lower current capacity to avoidover heating of the pump. it will be seen that the maximum throughputobtained with cathode cooling means is in the order of ten times greaterthan the maximum throughput obtained without cathode cooling means.

Referring now toFIG. 5 there is shown another embodiment of the presentinvention. The pump structure of this embodiment is substantially thesame as that of the apparatus shown in FIGS. 1 and 2 ywith the exceptionofthe particular cooling means utilized. In particular, the coppertubingfZS shown in FGS. 1 and 2 is replaced by the copper band 31 whichis brazed or otherwise secured in intimate contactwith the outer edge ofthe anode 12 and the high voltage lead-in 13. The leadin insulator 32outside the pump envelope has ns 33 which provide a greater surface fordissipating the heat which is conducted from the anode 12 by the lead-in13. The width of the copper band 31 is the same or less than the widthof the anode edge so that it does not affect the positioning of theanode or the path of the glow discharge. Thus, the copper band 31 andthe lead-in 13 provide a cooling means in intimate contact with theanode 12.

The operation of this embodiment and the function of the cooling meansis essentially the same as that described for the apparatus shown inFIGS. 1 and 2.

lt has been found in practice that cooling of the pump cathode has Lamuch greater' eiiect on improved through- FTM put capacity than doescooling of the pump lanode which has made it possible to achieve desiredmaximum throughput capacities in most cases by cooling only the pumpcathode. This fact has considerable signicance since cooling of thecathode does not present the many electrical insulation problemsencountered in cooling the high voltage anode.

However, it may be desirable in certain circumstances to provide a pumpwith intimate anode cooling alone or with both anode and cathodecooling.

The high throughput capabilities of the intimately cooled cold cathodepump can be advantageously used in special vacuum applications such asthe vacuum tube processing mentioned above. By controlling the processso that the large quantities of cathode conversion gas molecules areproduced in the pressure region of about l0*3 mm. Hg at the pump (i.e.,near the point of maximum throughput) the cooled pump will remove gasmolecules during this period at the same rate as would a pump of aboutten times its size without etiicient cooling. Furthermore, the moderatepumping speed of the cooled pump would be adequate to obtain the desiredultimate pressure in the tube during later periods of the vprocess whengas is being evolved at a much reduced rate. Thus, it will be apparentthat the utilization of the intimately cooled cold cathode pump of thepresent invention will allow the use of much smaller pumps in certainvacuum applications than was formerly possible.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and and not in a limiting sense.

What is claimed is:

1. An electrical vacuum pump apparatus including, an anode electrode, areactive cathode electrode spaced from said anode electrode, means forproducing and directing a magnetic field between said electrodes, avacuum compartment enclosing said anode and cathode electrodes, saidvacuum compartment being adapted for connection to a vacuum system,means for applying a potential to establish a glow discharge betweensaid anode land cathode electrodes so as to produce sputtering of saidreactive cathode electrode, means for cooling said apparatus, and saidcooling means being substantially enveloped by said vacuum compartment.

2. The apparatus according to claim 1 wherein said cooling means is inintimate contact with at least one of said electrodes.

3. The apparatus according to claim 1 wherein said cooling means ispositioned between said anode electrode and said cathode electrode.

4. The apparatus according to claim 3 wherein said cooling means is inintimate contact with at least one of said electrodes.

5. An electrical vacuum pump apparatus including, an anode electrode, areactive cathode electrode spaced from said anode electrode, a vacuumcompartment enclosing said anode and cathode electrodes, means forproducing and directing a magnetic eld between said electrodes, meansfor applying a potential to establish a glow discharge between saidanode and cathode electrodes so as to produce sputtering of saidreactive cathode electrode, means for cooling said apparatus, saidcooling means comprising an elongated heat-conducting member and beingin intimate Contact with at least one of said electrodes over asubstantial part of its length.

6. An electrical vacuum pump apparatus including, an anode member, areactive cathode member disposed opposite said anode member and beingspaced therefrom, a Vacuum compartment enclosing said anode and cathodemembers, means for producing and directing a magnetic field between saidanode and cathode members, means for applying a potential to establish aglow discharge between said anode and cathode members so as to producesputtering of said reactive cathode member, means for cooling saidreactive cathode member, and said cooling means being in intimatecontact with said reactive cathode member.

7. The apparatus according to claim 6 wherein said cooling meanscomprises tubing for circulating a cooling iiuid.

8. The apparatus according to claim 6 wherein said cooling means ispositioned between said reactive cathode member and said anode member.

9. The apparatus according to claim 8 wherein said cooling meanscomprises tubing for circulating a cooling fluid.

10. An electrical vacuum pump apparatus including, an anode membersubdivided into a plurality of cellular compartments, a reactive cathodemember disposed opposite the open end of said cellular compartments andbeing spaced therefrom, a vacuum compartment enclosing said anode andcathode members, means for producing and directing a magnetic eldcoaXially of said cellular compartments, means for applying a potentialto establish a glow discharge between said anode and cathode members soas to produce sputtering of said reactive cathode member, means forcooling said reactive cathode member, and said cooling means beingpositioned between said cathode member and said anode member.

1l. The apparatus according to claim 10 wherein said cooling means is inintimate contact with said cathode member.

12. The apparatus according to claim 11 wherein said cooling meanscomprises tubing for circulating a cooling iluid.

13. An electrical vacuum pump apparatus including, an anode member, areactive cathode member disposed opposite said anode member and beingspaced therefrom, a vacuum compartment enclosing said anode and cathodemembers, means for producing and directing a magnetic field between saidanode and cathode members, means for applying a potential to establish aglow discharge between said anode and cathode members so as to producesputtering of said reactive cathode member, means for cooling said anodemember, said cooling means comprising an elongated heat-conductingmember and being in intimate contact with said anode member over asubstantial part of its length.

14. An electrical vacuum pump apparatus including, an anode membersubdivided into a plurality of cellular compartments, a reactive cathodemember disposed opposite the open end of said cellular compartments andbeing spaced therefrom, a vacuum compartment enclosing said anode andcathode members, means for producing and directing a magnetic fieldcoaxially of said cellular compartments, means for applying a potentialto establish a glow discharge between said anode and cathode members,means for cooling said anode member, said cooling means comprising anelongated heat-conducting member and being in intimate contact with saidanode member over a substantial part of its length.

l5. An electrical vacuum pump apparatus including, an anode membersubdivided into a plurality of cellular compartments, a cathode memberdisposed opposite the open end of said cellular compartments and beingspaced therefrom, a vacuum compartment inclosing said anode and cathodemembers, means for producing and directing a magnetic field coaxially ofsaid cellular compartments, means for applying a potential to establisha glow discharge between said anode and cathode members, means forcooling said apparatus, and said cooling means being substantiallyenveloped by said vacuum compartment.

'16. An elecrical Vacuum pump apparatus including, an anode membersubdivided ino a plurality of cellular comparments, a cathode memberdisposed opposie the open en'd of said cellular compartments and beingspaced therefrom, a vacuum compartment enclosing said anode 5 andcathodemembers, means for producing and directing a magneic eldcoaxially 0f said cellular compartments, means for applying a poentialto establish a glow discharge between said anode and cathode members,means for cooling said cathode member, and said cooling means 10 beingin intimate contac with saidcathode member.

O 6:9 References Cited in the le of this patent UNITED STATES PATENTS

1. AN ELECTRICAL VACUUM PUMP APPARATUS INCLUDING, AN ANODE ELECTRODE, AREACTIVE CATHODE ELECTRODE SPACED FROM SAID ANODE ELECTRODE, MEANS FORPRODUCING AND DIRECTING A MAGNETIC FIELD BETWEEN SAID ELECTRODES, AVACUUM COMPARTMENT ENCLOSING SAID ANODE AND CATHODE ELECTRODES, SAIDVACUUM COMPARTMENT BEING ADAPTED FOR CONNECTION TO A VACUUM SYSTEM,MEANS FOR APPLYING A POTENTIAL TO ESTABLISH A GLOW DISCHARGE BETWEENSAID ANODE AND CATHODE ELECTRODES SO AS TO PRODUCE SPUTTERING OF SAIDREACTIVE CATHODE ELECTRODE, MEANS FOR COOLING SAID APPARATUS, AND SAIDCOOLING MEANS BEING SUBSTANTIALLY ENVELOPED BY SAID VACUUM COMPARTMENT.